In an MCM, a plurality of bear chips, namely IC chips which are not packaged, are mounted on a lead frame (in case of plastic package) or a base substrate (in case of ceramic package) and they are directly packaged in a package. This structure reduces inductances and capacitances because packages for respective chips are not required. The enhancement in the packaging density accomplished at the same time shortens wirings between chips, thereby to shorten the signal propagation delay time. Therefore, by employing the MCM, it is said that for a CPC module an operation at about 100 MHz is possible though it was impossible by a board mounting method. In such MCM, when high reliability is requested or anti-heating is required as in case of a CPU module of a work station and so on, it is general to employ a ceramic package, and when a low cost is intended, it is general to employ a plastic package.
FIG. 27 is a block diagram showing a general constitution of an MCM employing a ceramic package. As shown in FIG. 27, the MCM includes plural IC chips 203, a base substrate 202 as a printed wiring board on which the IC chips 203 are mounted and a package 201 accommodating the base substrate in its cavity part 201a. Each IC chip 203 and the base substrate 202 are connected by, for example, a wire bonding with such as an Al wire 204, flip chip bonding, or the like.
FIG. 28 shows an example of a pattern of the base substrate 205. The pattern of the base substrate includes a first type bonding pad 206 for connection between the base substrate 205 and the package, a second type bonding pad 208 for connection between each IC chip 207 and the base substrate 205, and a wiring pattern 209 connecting between these two type pads. As wirings of the base substrate 205, wirings between each bonding pad (hereinafter referred to as A-type pad) 206 for connection between the base substrate 205 and the package and each bonding pad (hereinafter referred to as B-type pad) 208 for connection between the base substrate 205 and the IC chip are patterned on the base substrate 205, as shown in FIG. 28. Also, as shown by 210 in FIG. 28, there are wirings between A-type pads, between B-type pads and among respective plural pads. The position on which each IC chip is mounted is shown in FIG. 28 with a broken line 207. When mounting is carried out by a wire bonding system, the bonding pad 206 is connected with an inner lead of the package by wire bonding and the bonding pad 208 is connected with a bonding pad of the IC chip by wire bonding.
FIG. 29 shows an example of a cross-sectional structure of the base substrate. In the base substrate, an insulating film 212 comprising SiO.sub.2, Si.sub.3 N.sub.4, polyimide or the like is formed on a substrate body 211 comprising silicon, ceramic, or the like, a wiring film is produced and patterned thereon, thereby forming a first wiring layer 213 comprising Al, Cu, Cr, or the like. Further, an inter-layer insulating film 214 comprising such as polyimide or SiO.sub.2 is formed as well as contact holes are produced thereat. Plural wiring layers and inter-layer insulating films are similarly formed alternately, and at last, a protection layer 218 is formed as well as apertures for bonding pads are produced. In order to inspect goodness of the completed base substrate, breakage of wires, short-circuiting between wirings, a quantity of capacitance accompanying with wirings, and a wiring resistance are detected as shown in FIG. 30. In order to carry out these, probes 243 and 244 of a probe card are contacted on an A-type bonding pad 241 for connecting the base substrate and the package and a B-type bonding pad 242 for connecting the base substrate and the IC chip, and turned-on electricity test between these probes 243 and 244 is performed.
In this prior art base substrate of MCM constructed as described above, when wirings on the base substrate are inspected, it was necessary to carry out an electrical inspection in a state where both of the A-type bonding pad and the B-type bonding pad are touched by probes of a probe card.
By the way, in an MCM on which a chip of 32 bit RISC (Reduced Instruction Set Computer) CPU, a chip of FPU (Floating Point Processing Unit) and four chips of cache memory are mounted, for example, 250 pads are required for peripheral A-type pads, for example, 420 pads are required for chip connecting B-type pads, as a total sum of 180 pads for the CPU, 80 pads for the FPU, and 160 pads for four cache memories because 40 pads are required for each cache memory. This means that the whole base substrate requires as much as 670 pads. As a result, the probe card that can probe all these pads at one time should inevitably have a lot of pins as well as should have probes for B-type pads positioned inside the substrate in addition those for A-type pads at the periphery of the substrate, resulting in extreme difficulty in producing a probe card which is depending on hand-making process. Even if such a probe card is actually produced, the production cost thereof could be extremely high, which would result in an increase in the cost of the MCM itself. Further, the probability of generating faulty contacts of the probe card onto the bonding pad during inspecting the substrate may become higher with a larger number of pins due to obstacles attached to the bonding pads. In addition, a tester needs a long time for inspecting all the pads, also leading to a high cost. When plural probe cards each probing a divided group of pads are provided so as to enable inspection of all the pads as a whole, the cost would also be higher because plural probe cards are required. In addition, such construction requires plural times attachment work of the probe card to the tester or to employment of plural testers for inspecting a single MCM, which results in also a high cost and a complicated work for inspection.